Follow-up to https://chromium-review.googlesource.com/c/v8/v8/+/941442.

"background" refers to a priority and is inappropriate to refer to
worker threads as many tasks posted to worker threads by v8 are in
fact high priority.

Also took advantage of this rename to make NumberOfWorkerThreads()
return an int instead of size_t. While it is never negative, int is
simpler and Google C++ style guide states to avoid unsigned integers in
such cases (ref. "On Unsigned Integers" @
https://google.github.io/styleguide/cppguide.html#Integer_Types).

The Chromium embedder for that call provided an int which was converted
to size_t for this override and most often casted back down to int on the
v8 side, adding churn, and readability overhead.

R=ahaas@chromium.org

Bug: v8:7310
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ib5280df73d2846b111d985be65a10b049995ea6a
Reviewed-on: https://chromium-review.googlesource.com/941944
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51662}